A cased borehole is a borehole lined with a casing that has been cemented in the borehole so that the annulus between the outer surface of the casing and the inner surface of the borehole is filled with set cement. The casing is filled with liquid used to displace the cement out of the casing and into the annulus, before the cement sets. The liquid in the casing is so dense that fluids are prevented from entering into the casing.
In order to obtain a sample of the formation fluid from the formation layer, the casing wall is perforated in a predetermined interval within that formation layer. The tool used to create the perforations, is a perforating gun. This is an elongated body provided with a plurality of outwardly directed charges. The charges are arranged at different locations along the body oriented in different directions, and they can be activated electrically or mechanically. The charges are so designed that each charge on activation produces a perforation including a perforation tunnel that extends through the wall of the casing into the formation surrounding the borehole. The perforating gun can be lowered into the cased borehole by means of for example a wireline.
In order to obtain a fluid sample, the perforating gun is lowered to the predetermined depth and the charges are activated to create a plurality of perforations. The liquid present in the casing prevents formation fluid from entering into the casing.
Then a sampling tool is lowered into the cased borehole by means of for example a wireline. The sampling tool comprises a central conduit having an inlet and a discharge, a fluid sample container opening into the central conduit, and a system for discharging fluids from the central conduit and for moving fluids into the fluid sample container. The sampling tool is further provided with an upper and a lower packer arranged at either side of the inlet of the central conduit, wherein the discharge opens below the lower packer. The distance between the upper and the lower packer is greater than the height of the perforations.
The sampling tool is so positioned that the upper packer is located above the perforations and the lower packer below the perforations. Then the packers are set to seal off a sampling space between the packers into which all the perforations open.
The system for discharging fluids from the central conduit and for moving fluids into the fluid sample container includes a pump 58 (FIG. 1). The pump 58 is activated to remove the liquid from the sampling space. The time required to remove the liquid from the sampling space is substantially equal to the volume of the sampling space divided by the pump rate.
The pump is further activated and the fluid that enters into the central conduit is now moved into the sample container. Once the sample container is filled, it is sealed off and the sampling tool is retrieved from the borehole.
At surface the sample container is brought to a laboratory for further analysis. This analysis is important because it can give an answer to the question whether or not the formation fluid is a valuable hydrocarbon.
Unfortunately, the sampled fluid need not always represent the formation fluid. For example when the cement in the annulus does not completely fill the annulus, there is a channel with a low resistance to fluid flow. Thus fluids from the channel will preferentially be drawn into the sampling space.